1. Field of the Invention
The present invention relates to metabolic gas monitoring apparatus and methods, and in particular, to apparatus and methods for measuring the metabolic rate of patients.
2. The Prior Art
Prior art systems for monitoring the metabolic rate of human patients are well-known. In such systems, the amount of oxygen (O.sub.2) and carbon dioxide (CO.sub.2) are measured in both the gas inspired by the patient and the gas expired by the patient, and the amounts of oxygen consumed and carbon dioxide produced are calculated so as to determine the metabolic rate of the patient. Oxygen and carbon dioxide sensors are typically used for this purpose. The amount of oxygen consumed and the amount of carbon dioxide produced by a living subject reflects the nutritional and metabolic status of the body. Accurate measurement of these parameters may thus be extremely helpful in the treatment of various patients.
The measurement of the oxygen consumption rate and the carbon dioxide production rate in an individual serves, for example, as an indicator of relative changes in cardiovascular function and tissue perfusion, which must be carefully monitored in critically ill patients. Typically, there is an increase of catabolism of protein and an associated loss of body weight resulting from the breakdown of tissue required to supply energy for the dramatic metabolic requirements in critically ill patients, thus providing further reason for careful monitoring of the metabolic rate. In burn patients, the metabolic rate may increase by fifty to three hundred percent (50%-300%).
Additionally, measurement of a patient's metabolic rate is useful in calculating the energy expenditure for a patient with regard to surgery, infection, or injury. Careful measurement of the metabolic rate can provide an accurate basis for formulating a dietary plan for the patient, and thus ensure that the patient's caloric intake is properly coordinated to avoid lipogenesis and other adverse physiological consequences of excess caloric consumption. Also, measurement of the metabolic rate is useful in determining a patient's response to exercise, and is often used in stress test measurement.
One disadvantage of many prior art systems for measuring the metabolic rate is that typically the apparatus employed are complex and expensive, since the apparatus are not combined into a single compact unit. These systems are generally cumbersome to move and require expert attendance for accurate operation.
Another problem encountered in prior art systems for measuring the metabolic rate of a patient is the presence of water vapor in the inspired and expired gas. Gaseous samples of inspired air typically have a water vapor partial pressure of about 0-25 torr. Moreover, in patients receiving ventilatory support in which the inspired gas is humidified, the water vapor pressure typically varies between 0 and about 47 torr. Gaseous samples of a patient's expired gas typically have a water vapor partial pressure of about 47 torr. Since water vapor may be detected by the oxygen sensor and may adversely affect the accuracy of the oxygen sensor, the prior art has sought to remove water vapor from the inspired and expired gas.
Moreover, another detrimental effect of water vapor on respiratory gas analysis is that the partial pressure of the water displaces the analyzed inspired or expired gas, thereby further resulting in inaccurate readings. Another associated problem is the error introduced if the water vapor concentration in the inspired gas is not equivalent to the water vapor concentration in the expired gas. Such equalization is needed to cancel out the effects of water vapor when the oxygen concentrations of the inspired and expired gases are compared.
One attempt to remove water vapor from the expired gases prior to analysis resulted in physically drying the expired gas, for example, by introducing the expired gas into a desiccator. One system has been developed using a desiccator filled with calcium sulfate (CaSO.sub.4) as the drying agent. Such desiccator systems experience at least two significant problems: (1) the drying agent must be carefully watched and replaced on a regular basis, and (2) the volume within the desiccator required to perform the drying makes for increased dead space within the system and thus results in a longer "washout time" for measuring changes in gaseous composition. As used herein, the term "washout time" refers to the amount of time which is needed for a unit of gaseous sample to wash out or displace the gas already within the system.
Washout time is an important factor in monitoring changes in the oxygen and carbon dioxide concentrations within the inspired and expired gases. Where large total volumes or dead volumes are present within a metabolic gas monitoring system, corresponding large washout times are created, resulting in decreased ability to quickly and accurately measure changes in the composition of the inspired and expired gases. Indeed, in prior art systems employing a desiccator to dry expired gases, a breath-by-breath analysis of the patient's expired gases is extremely difficult, if not impossible. Again, this is because the long washout times in such systems do not allow for the dynamic response to changes in the oxygen and carbon dioxide concentrations in breath-by-breath analyses of expired gas. Thus, large total volumes and dead volumes within such prior art systems has resulted in less sensitivity to changes in the composition of the gases analyzed and less accurate measurements of the oxygen and carbon dioxide components of the gases.